For the transmission of payload data over packet-switched communication networks said data is as a rule preceded by header data (abbreviated to header) which is essential for the transport of the data packet in the communication network. Header in this case refers to the part of the data packet containing not payload data but various administration data, e.g. address, packet number, sender id. packet status as well as data for error detection and error correction (e.g. checksum CRC).
With narrowband data streams this header data can amount to a multiple of the payload data to be transmitted. Header compression methods are therefore frequently used for such data streams, such as PPP dial-up connections for example. A compression of the header data is above all desirable for a transmission over wireless connections, since extensive volumes of header data of IP protocols impose a particular load on mobile radio channels.
IP is responsible for the connectionless transport of data from a sender, over one or more communication networks, to a recipient, with no error detection or correction being performed. This means that IP does not concern itself with harmful or lost packets. The abbreviation IP here stands for Internet Protocol, a protocol of the TCP/IP-family on layer 3 of the OSI reference model. IP is used by a number of protocols located in higher layers of the OSI reference model, primarily by TCP (Transfer Control Protocol), but also by UDP (User Datagram Protocol).
A known protocol header compression method can be found for example in S. Casner and V. Jacobson, “Compressing IP/UDP/RTP Headers for Low-speed Serial Links”, Network Working Group, Request for Comments: 2508; (available for inspection on the Internet at http://www.ietf.org/rfc/rfc2508.txt?number=2508).
This proposes different encodings for different protocols. Thus separate encoding is undertaken for RTP headers of end-to-end connections, whereas in the case of a link-to-link-connection a common compression of RTP/UDP/IP headers is possible.
Provision is made here for encoding with three different code stages:                a full header (FH)        a first order difference (FO), namely the description of two consecutive headers by a variable length code and        a second order difference (SO), i.e. the transmission of the differences of two FO headers.        
For a compression of TCP headers, after an uncompressed header has been transmitted once, fields which change are described by a differential encoding in order to reduce their size. In addition fields which change are completely eliminated by calculating changes on the basis of the length of a packet. This is based on the knowledge that around half of the bytes in IP and TCP headers remain unchanged for the duration of a connection.
One of the disadvantages of this method is that these known compression methods are specific to certain protocols and only function for these. For the different variants of the encoding of individual protocols or of more than one protocol at the same time different encoders must be available, which increases the effort involved and adversely affects economy. To enable the new protocols to be compressed, it is necessary to define new methods, implement them and update the network components.
Other approaches tackle the problem without prior knowledge about the protocols to be compressed and consider a data packet, consisting of header data and payload data, as a byte or bit stream and encode this differentially. One disadvantage of these methods is that the checksums frequently occurring in the header data exhibit a high entropy and cannot be compressed at all or can barely be compressed. This header data is always transmitted in each case and needs a wide bandwidth, regardless of the payload data to be transmitted.